1. Field of the Invention
The present invention relates to a circuit for hands free talking connected between a general telephone or a mobile phone or the like which enables simultaneous transmission and reception, i.e., simultaneous talking, and a microphone and a speaker so as to realize half-duplex hands free talking.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional circuit for hands free talking. In FIG. 1, reference numeral 1 means a transmit signal input terminal into which a transmit signal from a microphone is inputted, and 2 means a transmit signal output terminal from which the transmit signal is outputted to a telephone. Reference numeral 3 means a receive signal input terminal into which a receive signal from the telephone is inputted, and 4 means a receive signal output terminal from which the receive signal is outputted to a speaker. Reference numeral 5 means an integrated circuit (hereinafter referred to as IC) for use in a speaker phone to realize half-duplex talking by controlling attenuation of the transmit signal and the receive signal. For example, IC M34118 (available from Motorola, Inc.) may be used as the IC 5. Further, reference numeral 6 means a speaker amplifier to amplify the receive signal which is outputted to the speaker.
In addition, in the IC 5, reference numeral 10 means a transmitting variable attenuator (S. V. ATT) to enable variation in the attenuation of the transmit signal, 11 means a receiving variable attenuator (R. V. ATT) to enable variation in the attenuation of the receive signal, and 12 is an attenuator controller to control attenuation of the transmitting variable attenuator 10 and the receiving variable attenuator 11. Reference numeral 13 means a level detector to detect a signal level of an input terminal of the transmitting variable attenuator 10, 14 means a level detector to detect a signal level of an output terminal of the transmitting variable attenuator 10, 15 is a level detector to detect a signal level of an input terminal of the receiving variable attenuator 11, and 16 is a level detector to detect a signal level of an output terminal of the receiving variable attenuator 11. Reference numeral 17 means a microphone amplifier to amplify the transmit signal input into the transmit signal input terminal 1, 18 means a background noise monitor (S. N. Mon) to distinguish noise from voice on the transmitting side, and 19 is a background noise monitor (R. N. Mon) to distinguish noise from voice on the receiving side.
The positioning of such a hands free circuit with respect to a telephone and a microphone and loudspeaker is shown in FIG. 1A wherein a hands free circuit 100 is positioned between a telephone 102 and a loudspeaker 106. The operation of the hands free circuit 100 to enable hands free transmission of audio signals from the microphone 104 and hands free reception of signals at the loudspeaker 106 through the telephone 102 is well known in the art. The present invention relates to an improvement in the circuit for hands free talking 100 which is connectible between a telephone 102 and speaker 106 and microphone 104.
A description will now be given of the operation. An aural signal input through the microphone which is connected to the transmit signal input terminal 1, is amplified by the amplifier 17 in the IC 5, and is thereafter input into the transmitting variable attenuator 10. Concurrently, the output of the amplifier 17 is also input into the level detector 13 through a capacitor and a resistor. The transmit signal passing through the transmitting variable attenuator 10 is outputted from the transmit signal output terminal 2 to the telephone. At the same time, the output of the transmitting variable attenuator 10 is also input into the level detector 14 through a capacitor and a resistor.
On the other hand, the receive signal from the telephone is inputted into the IC 5 through the receive signal input terminal 3, and is inputted into the receiving variable attenuator 11 of the IC 5. Concurrently, the receive signal is also input into the level detector 15 through a capacitor and a resistor. The receive signal passing through the receiving variable attenuator 11 is inputted into the amplifier 6 to be amplified, and is outputted to the speaker through the receive signal output terminal 4. At the same time, the output of the receiving variable attenuator 11 is also input into the level detector 16 through a capacitor and a resistor.
The four level detectors 13 to 16 are connected to the attenuator controller 12. The attenuator controller 12 compares levels at four positions to control the attenuation of the transmitting variable attenuator 10 and the receiving variable attenuator 11 based on results of the comparison. This control equivalently opens either a transmitting switch or a receiving switch to set the circuit in a transmitting mode or a receiving mode. In the operation, it is possible to distinguish the voice from ambient noise by the background noise monitors 18 and 19 mounted on the transmitting side and the receiving side. Such a half-duplex talking system can avoid howling generated due to telephone line echo or acoustic coupling from the speaker to the microphone.
Arts relevant to the conventional circuit for hands free talking are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 60-62733, 61-262325, and 64-20767.
Since the conventional circuit for hands free talking is provided as set forth above, there are several problems as follows: Delay of detection may cause delay of switch changing, resulting in a truncation of the beginning portion of a word. At a time of simultaneous talking, the operation of the transmitting variable attenuator 10 and the receiving variable attenuator 11 become unstable so that a speech level becomes unstable. Further, gain of the respective level detectors 13 to 16 is determined on the basis of a value of each resistor connected to each pin of the IC 5, and it is difficult to set the transmission and the reception in balance. Erroneous balance may cause tendency to be easily switched to one of a transmission mode and a reception mode, and the other mode may be sacrificed. Additionally, the IC is an expensive IC as is for the speaker phone.